The present invention relates to bi-phase code decoding systems for decoding bi-phase codes contained in video signals and, more particularly, to bi-phase code systems which can reliably decode bi-phase codes contained in video signals even when the speed or rotation of a laser disc (LD) is unstable.
FIG. 3 shows a bi-phase code decoding system of the pertaining type. A motor control circuit 31 of a laser disc player (hereinafter referred to as LDP) makes a control for stabilizing the rotation of a laser disc (hereinafter referred to as LD). After an LD servo circuit 32 outputs a stable rotation signal by detecting that the rotation of the LD has been stabilized, the bi-phase code is decoded by sampling data. The system includes an LD data reader 33, a horizontal sync counter 34, and a bi-phase code decoder 35. The bi-phase code decoder 35 extracts the decoded code.
The LD data reader 33 reads out the LD data, and supplies the read-out horizontal sync signal (HSYNC) and vertical sync signal (VSYNC) to the horizontal sync counter 34 and the video signal to a bi-phase code decoder 35. The horizontal sync counter 34 receives the horizontal and vertical sync signals and counts the horizontal sync signal. The horizontal sync counter 34 outputs a decoding start signal to the bi-phase code decoder 35 when the horizontal sync signals are horizontal scan lines including a bi-phase code.
The bi-phase code decoder 35 receives the decoding start signal from the horizontal sync counter 34 and outputs sampling data which is obtained by the sampling in synchronism with the horizontal and vertical sync signals and a clock signal at a higher frequency than the frequency of the bi-phase code.
As shown in FIG. 4, rise and fall edges of the bi-phase code data are judged. When the code data edge is the rise edge, it is decoded as "1". When it is the fall edge, it is decoded as "0". When the same data is judged continuously, dummy data between adjacent edges of the intrinsic data are also continuous. Therefore, a deviation of the sampling timing due to unstable rotation of the LD, would result in sampling of a dummy edge.
As shown above, the prior art bi-phase code decoding system has a problem that a deviation of the sampling timing due to unstable LD rotation may give rise to an opportunity of sampling a dummy edge, resulting in erroneous judgment or failure of the judgment.